1. Field of the Invention
The present invention relates to a servo motor for outputting a control signal appropriate for performing rotary control of a servo motor having a PID (Proportional Integral Differentiation) controller, and more particularly to a limiter circuit of a servo motor control apparatus which can perform limit control in the forward direction or reverse direction, or both the forward and reverse directions.
2. Description of the Prior Art
Generally, a PID controller used for automatic control of an AC servo motor, as depicted in FIG. 1, comprises a proportional amplifier 100, an integrating circuit 101 and a differentiation circuit 102.
In the PID controller of the AC servo motor 1, the desired value for servo motor control outputted from a desired value command (not shown) and the actual displaced quantity of the servo motor 1 as driven by the desired value are added at an adder 99. The output signal of the adder 99 is amplified at the proportional amplifier 100. The output signal of the adder 99 is integrated in order to zero the errors of the AC servo motor during the driving. The output signal of the adder 99 is differentiated in order to eliminate the delaying elements of the AC servo motor.
Then, each output signal of the proportional amplifier 100, integrating circuit 101 and differentiation circuit 102 is added at an adder 103 for controlling the AC servo motor 1.
The AC servo motor comprising the PID controller can be operated at a speed the user has preset. The revolution of the servo motor can be adjusted for constant velocity or for appropriate output by utilizing the PID controller as desired to display a rated output power.
When an operation is performed in a condition under which the predetermined revolution is not reached due to an overload even with maximized output of the motor, or when the operation of an instantaneous return to a normal load is performed under an overload condition, or when a start is performed under a condition of the motor being stopped, the PID controller, in particular the integral circuit 101, accumulates errors which show deviations from the desired value. As a result, the output value of the integral signal value accumulated on the integral circuit 101 increases, causing a problem in that the servo motor running excessively.
Furthermore, a great deal of time on the order of hours is spent in order for the output of the increased integral signal value to return to the original condition. This occurs particularly when the overload returns to the normal load or when the motor has been started from a standstill condition. Practical problems in the use of the conventional PID controller.
In order to compensate for the aforementioned problems, as illustrated in FIG. 2, a pair of Zener diodes ZD is connected to the PID controller in two directions for limiting the output in relation to the Zener voltage. However, the Zener voltage of the Zener diodes ZD, once set, cannot be varied when the output limit value changes.
To solve the aforementioned problems, there is provided a rotary control method in Japanese laid open patent application No. Hei 1-94412 as depicted in FIG. 3.
In the rotary control method, the integrating circuit 101 includes a PID controller connected to a limiter 110. The limiter 110 is divided into 2 limiter circuits 105, 106 for controlling the main and auxiliary integral signal values respectively in order to prevent the integral signal value of the integrating circuit from exceeding a limited value.
The conventional rotary control method can freely change the output value of the PID controller within predetermined ranges for control by sliding the variable contacts of variable resistors VR1, VR2. However, the operating condition of the motor changes too often during operation. Consequently, a problem has resulted of being unable to set the output limit value in order to adapt to instantaneously-changing situations.